Ever since realising that the stock Apple earbuds and later, Skullcandy, are absolutely rubbish (hey, don't judge too harshly ), I've been on the lookout for portable 'phones that are affordable and actually sound good while on the go; whether they be over-the-head portables or IEMs. I am a basshead and like to have a thumping bass-line pervading my music and so, I got 3 of the better thought of 'bass heavy' IEMs: Denon AH-C551, Klipsch S4 and Hippo VBs (all within more or less $80, so fairly affordable) to satisfy my bass fuelled musical needs. The following thoughts are IMO, so you have fairly good disclaimer right there :

PHOTOS
Sorry for not taking photos personally, my camera's FUBAR:ACCESSORIES
All the phones come with a case, all of very good quality, which can accommodate the earphone and as a bonus, my Clip+ too. The most practical case, for my purposes is the Hippo VB case as it is pretty flat, zippered and doesn't bulge out while in the pocket. The C551 case, while somewhat smaller, is a bit chubbier than the VB case. The S4 metal case, though providing better safety, is kind of wobbly and opens easily.

Although not in the above pics, all 3 come with shirt clips and the regular assortment of small, medium and large silicon single flanged tips and the S4 includes an additional double flanged set, which is on the small side, so it might not useful to users with large ears.

A nice touch from the S4 is the earwax cleaning tool, which is useful. The C551 come with a 6.3mm adapter which is useful for home listening.

DESIGN, BUILD, COMFORT
The design of the C551 and Hippos is simple and minimal; the housings are matte black anodized aluminium with laser engraved fonts and look nice. The S4 on the other hand looks very flamboyant and the housings, although entirely made of plastic, don't feel cheap or nasty. All the 'phones have good stress relief.

Buildwise, the C551 easily dominates over the VBs and the S4s. They are built like tanks! I've used them for almost a year now, in varyingly worrying conditions (home, burning desert, very wet monsoons) and they have still pulled through without any disturbing physical damage. The only complaint against the build is the split cable. It feels far too long for an IEM and gets tangled very easily; the slider at the Y-split reduces this to some extent, however.

The Hippo VBs have decent cable quality and the 3 exchangeable bass ports at the back of the earbuds, to control the bass, are screwed on pretty tightly and have a nice, safe fit.

The S4 build quality is probably the most rubbish out of these 3. The cable feels far too thin and again, gets tangled too easily.

Isolation on the C551 from outside noise is average with the provided silicon tips, below average on the Hippos and quite good on the S4. The tips are all a bit thin, but the IEMs can be used with other tips such as Comply's and Sony Hybrids. In fact, I use the C551s with the Hybrids, which improve isolation.

Microphonics are really bad, with all the phones exhibiting annoying cable noise. The C551 and VBs can be worn over the ears, which reduces cable noise significantly. However, the S4 cannot be worn this way and that's where the included shirt clip comes in handy.

Comfortwise, the C551s can be slightly uncomfortable- if using the stock tips after about an hour's worth of listening; however for me, this was corrected by using the Hybrids. If the Hybrids are not for you, the C551 can be used with Comply's too, but they are just too expensive and in my opinion a complete ripoff, YMMV. Hippo VBs are very comfortable right out of the box (although I've heard and read about complaints from other users, but for me they are pretty good straight away) and even the S4s with the round oval tips feel very comfortable.

SOUND
As I mentioned earlier, I am a basshead and bass was the yardstick while measuring these IEMs against each other. However, it wasn't just muddy, uncontrolled bass I was after, otherwise I would have gone with Skullcandy and you wouldn't be here reading this. I wanted there to be a level of clarity too and the bass to be there as an undercurrent and not separate sounding, kinda like a really good quote in a paragraph with a really thick underline to make it stand out. Lame analogy, I know, but the best I could think of right now. Anyhoo..

Denon AH-C551
These are very capable with regards to bass. Very, very capable. And it matches what I've said above. The bass is not a separate entity, its part of the whole song. What I mean to say is that these are warm with very smooth mids and upper mids. However, the highs leave a lot to be desired; for example, cymbals and some background vocals just blend together. Its not very distracting while on the move however, and if you're not very picky with regards to this, then these are very good sounding earphones for the price. Although I dont really believe in burn-in, the C551s have changed their sound signature over time and I have required to EQ them in recent months to get the best out of them. All Hail Rockbox!!

Hippo VB
The main feature of the VBs that attracted my attention is the exchangeable bass-ports (Variable Bass). These little plates let you adjust the amount of bass and come in three flavours- no dot, 2 dots and 3 dots. The no dot plate is pretty crappy IMO and makes the VBs sound harsh and sibilant while the 3 dots plate, although big on bass, makes the VBs sound a bit muddy. The 2 dots plate is where the sweet spot is, with the right amount of bass while maintaining a decent amount of clarity throughout. Mids are good and the highs are better than the C551, although there can be slight traces of sibilance. Again, as with the C551, these have changed their sound signature since I got them a few days back. I've used them for about 24 hours now, and their bass seems to have settled down, but mids and highs are more or less the same. Along with the decent build quality and excellent SQ, these are IMO the best value for money IEMs you can get for $80.

Klipsch S4
This is all I can say about the S4:
Bass: VB>C551>S4 (more than the C551 actually, but not as controlled)
Mids: VB>C551>S4
Highs: VB>C551>S4 (dear God, the sibilance!! although this wasn't apparent when I first got them)
Build Quality: C551>VB>S4
Don't get me wrong, there's not much amiss with the S4 (apart from the sibilance and the build quality) and given as an only choice is a very good IEM for the price. However when stacked up against the other two, there's very little reason to pick the S4 over a VB or the C551 as it offers very little value for money against either of them.

CONCLUSION
Phew, long read eh ? I was basically trying to imitate dfkt from here and hence the similar pattern.

So finally, what do I want to say: Which is the best basshead IEM from the above choices? Personally, the Hippo VB, hands down. Jaben has done a freakin sweet job with these and the plenty of happy customers, here on Abi, Jaben's own forums and Head-Fi will attest to the really good SQ and overall value for money of these IEMs.
I really like the Denons as well and if not for the distorted and weak highs would have tied or even surpassed the VBs due to their better build quality.
The S4s were a serious disappointment as I had heard so many positive reviews of them that my expectations were probably higher than usual. The only good thing that came of buying these was that my GF had broken her Senn CX500s and wanted replacements. Somehow she loves the S4, doesn't think them sibilant and accuses me of being too picky . Which leads me to saying what I've said earlier: this is IMHO, YMMV.